The Old English language preserves the system of declension only for numerals from 1 to 3. Here is the list of the cardinal numerals:
1 ān
20 twentig
2 twā
21 twentig ond ān
3 þrīe
30 þrītig
4 fēower
40 fēowertig
5 fīf
50 fīftig
6 six, syx, siex
60 siextig
7 seofon, syofn
70 siofontig
8 eahta
80 eahtatig
9 nigon
90 nigontig
10 tien, týn
100 hundtēontig, hund, hundred
11 endlefan
110 hundælleftig
12 twelf
120 hundtwelftig
13 þrīotīene
200 tū hund
14 fēowertīene
1000 þūsend
15 fīftīene
2000 tū þūsendu
1 ān is declined just like a strong adjective, can be only singular, but has masculine, neuter and feminine genders. It is the source of the future indefinite article 'a, an' in Modern English. So 'a house' in fact means "one house", here -n disappeared before a consonant.
2 twā:
Masculine
Neuter
Feminine
Nominative
twegen
tū, twā
twā
Genitive
twēgea, twēgra
Dative
twǣm, twām
Accusative
twegen
tū, twā
twā
No number can be changed for this numeral, and originally this numeral was dual, which seems natural.
3 þrīe:
Masculine
Neuter
Feminine
Nominative
þrīe, þrī, þrý
þrīo, þrēo
þrīo, þrēo
Genitive
þrīora, þrēora
Dative
þrīm
Accusative
þrīe, þrī, þrý
þrīo, þrēo
þrīo, þrēo
The numeral begen, bū, bā (both) is declined the same way as twā and is also dual.
Ordinal numerals use the suffix -ta or -þa, etymologically a common Indo-European one (*-to-).
1 forma, fyresta
14 fēowertēoþa
2 ōþer, æfterra
15 fīftēoþa
3 þridda, þirda
16 sixtēoþa
4 fēorþa
17 siofontēoþa
5 fīfta
18 eahtatēoþa
6 siexta, syxta
19 nigontēoþa
7 siofoþa
20 twentigoþa
8 eahtoþa
30 þrittigoþa
9 nigoþa
40 fēowertigoþa
10 tēoþa
50 fīftigoþa
11 endlefta
12 twelfta
100 hundtēontiogoþa
13 þreotēoþa
The two variants for the word "first" actually mean different attributes: forma is translated as "forward", and fyresta is "the farthest", "the first". Again double variants for the second nominal mean respectively "the other" and "the following".
Mainly according to Old English texts ordinal numerals were used with the demonstrative pronoun þā before them. This is where the definite article in 'the first', 'the third' comes from.